As a anode body of an electrolytic capacitor, a wound body of surface-roughened aluminum foil, a single layer body or multi-layered body of surface-roughened aluminum thin plate, a porous sintered body of tantalum powder and so on are much used.
As a material of the anode body, niobium is also gathering attention.
Niobium that, similarly to tantalum, belongs to 5A group in the periodic table is a metal close in the physical properties to tantalum and has various advantages such as, in comparison with tantalum, the specific gravity being smaller, the reserves being more abundant, the price per kg being less expensive and so on. Accordingly, various attempts have been carried out to utilize niobium as the anode material. However, there are many problems in that in an electrolytic capacitor that uses niobium as the anode material, the leakage current is likely to increase, the aging treatment (an operation to make defect portions of a dielectrics film insulative by applying a direct current voltage in accordance with the polarity of the capacitor for a long period of time) to reduce the leakage current is difficult to demonstrate desired effects, a value of electrostatic capacitance is likely to fluctuate according to a direct current bias voltage and so on. These problems cannot be overcome by simply applying the technologies that are used in the tantalum anode body. As far as the present inventors know, the niobium anode body is not yet commercialized.
On the other hand, as a cathode material of an electrolytic capacitor, in place of a conventional electrolyte solution, a solid electrolyte such as a conductive oxide such as manganese dioxide and so on, and an organic semiconductor such as TCNQ complex and so on has become to be frequently used. Furthermore, recently, conductive polymers such as polypyrrole, poly-thiophene and so on have been also put into practical use. Anyhow, the anode material is limited only to aluminum or tantalum.
As means for forming the conductive polymer layer as the cathode material with a certain degree of thickness, (1) a method of repeating chemical polymerization several times, (2) a method of forming, on a thin conductive polymer layer formed by a chemical polymerization process, by means of electrolytic polymerization, a relatively thick conductive polymer layer (JP-B-04-74853) and so on are known.
Here, the chemical polymerization means to oxidation-polymerize a polymerizing monomer without using energizing means but using an action of an oxidant. The electrolytic polymerization means to oxidation-polymerize a polymerizing monomar by use of the energizing means.
The methods of forming a conductive polymer layer according to the (1) and (2), respectively, have merits and demerits depending on combinations of materials of anode body and constitutions thereof and so on. However, when taking the easiness in forming a dense conductive polymer layer, the easiness of controlling a polymerization reaction, the long pot-life of a polymerization solution and so on into consideration, the later (2) (a method that uses the chemical polymerization and the electrolytic polymerization in combination) is more advantageous.
In recent years, technology as to niobium powder for capacitors has demonstrated dramatic improvements. That is, the CV product (a product of electrostatic capacitance C per unit mass obtained by forming a dielectrics film and chemical conversion (anodic oxidation) voltage V for forming the dielectrics film) was improved and proved to be hardly different from that of the tantalum powder; and, the purity of the niobium powder was remarkably improved such as that an absorbed oxygen concentration was reduced from the conventional several tens thousands ppm to several thousands ppm and so on.
In this connection, researches toward the practical applications of niobium electrolytic capacitors are in boom. In particular, since niobium hates high temperatures, combinations with conductive polymer cathodes that allow forming by processing and operating at relatively low temperatures are targets of flourishing researches because these are considered the most shortest crosscut.
However, as the researches of the niobium electrolytic capacitors progress as mentioned above, the differences from the case where the tantalum anode body is used are gradually revealed, and it has become obvious that simple transfer of the conventional technology of the tantalum electrolytic capacitors is far from manufacturing practically applicable ones.
Problems that the present invention is to solve are cited as follows.
Firstly, when a dielectrics film layer and a conductive polymer cathode layer are formed on a surface of a anode element that is made of a niobium sintered body, even when the conditions the same as that in the case where a tantalum sintered element is used are applied to form the respective layers, the leakage current becomes very large, that is, an almost short-circuited state is caused, resulting in being incapable of, in many cases, carrying out the aging treatment.
Furthermore, granted that dielectrics film layer formation conditions and conductive polymer layer formation conditions that allow applying the aging to some extent are found, the electrostatic capacitance occurrence rate (a ratio of the electrostatic capacitance measured after the formation of the conductive polymer layer to that measured in a conductive aqueous solution before the formation of the conductive polymer layer) is low. That is, different from the case where the tantalum sintered body is used, it is very difficult to form the conductive polymer layer to a center portion of the niobium sintered body.
As to the capacitance occurrence rate, in the case of the tantalum sintered body being used, whether the conductive polymer layer is formed according to the chemical polymerization method or to a method that combines the electrolytic polymerization method therewith, substantially 85% or so can be relatively easily achieved. On the other hand, in the case where the niobium sintered body is used, it is not easy to conquer a wall of from 20 to 30%.